1. Field of the Invention
This invention relates to devices for monitoring bridges and similar large engineered structures which vibrate in response to environmentally induced excitations such as traffic or wind. More specifically, the invention relates to monitoring the motion and structural integrity of such structures with multiple vibration sensors by collecting data from all of the sensors substantially simultaneously in order to determine the response mode shapes of the vibrating structure.
2. Description of Related Art
The structural integrity of large steel and concrete structures, such as bridges, is a subject of continuing concern. One approach to monitoring the structural health of bridges has been to supplement visual inspection with vibration monitoring using linear or angular motion sensors. Such systems are shown in U.S. Pat. No. 4,956,999 issued to Bohannan et al., on Sep. 18, 1990 (linear sensors) and U.S. Pat. No. 4,164,149 issued to Okubo on Aug. 14, 1979 (angular sensors).
One problem with such data collection systems is that although they use multiple sensors, the data from each sensor is collected slowly or at different times. As a consequence, although the collected data may be sufficient to determine the precise motion of any individual monitored location on the structure, it is not possible to correlate the data from one monitored point with the data from another point so as to determine how one portion of the bridge is moving relative to another portion.
In most earlier vibration data collection devices, multiple sensors have been used so that at least one sensor will be located near a potential failure site. At this location, it was believed that the maximum change in vibrational characteristics would occur. However, in the present invention, data is collected at multiple sites so that a global analysis can be performed. It has been found that the shape of the vibration modes for the entire structure will change significantly due to potentially catastrophic failures in the structure which may not be picked up by prior art monitoring devices.
Prior art systems have not recognized the value in collecting the data from multiple sensor locations simultaneously. Most previous systems have used single channel data collection methods where a single accelerometer or angular sensor is monitored for a relatively long period of time, i.e., for one or more cycles of the vibration frequency of interest.
Even where multiple sensors are used and the monitoring can be switched between them, the long monitoring time for each sensor means that the position of the first sensor will have changed significantly before the second or any subsequent sensor is monitored. The problem of determining exactly how the bridge is moving and responding to an excitation is made even more difficult when the excitation is random. Such is the case with environmentally induced excitations, e.g. traffic induced bridge vibration, where the time of the excitation, the amplitude and the location of the excitation point all vary significantly. This makes it impossible to compare or corrrelate data from different points in prior art systems of data collection.
Some systems avoid this problem by using a known impulse excitation, such as a hammer blow or variable frequency vibration driver to excite the vibrations instead of monitoring environmentally induced vibrations, but this is not always practical or desirable.
Another problem with some prior art systems has been that they convert analog data from the motion sensors to digital data at too early a point, or they multiplex data to reduce cabling costs and fail to use anti-aliasing procedures. This can lead to errors in the data analysis such as phantom vibration peaks.
Consequently, one object of the present invention is to provide an apparatus that collects vibration data from multiple points on a vibrating structure, substantially simultaneously up to a maximum frequency of interest.
Another object is to provide a method of analyzing the collected data and calculating the vibration mode shapes of the monitored structure.
Still other objects and advantages of the invention will be obvious from the specification.